1. Field of the Invention
The invention concerns aluminum coated metallic sheets.
2. Background of the Invention
The application of a metal coating based on aluminum onto a sheet is a means that is routinely used to protect a sheet made of steel against corrosion, notably in the case where the temperature of use of this sheet exceeds approximately 400.degree. C. The thickness of the metallic coating in question is generally 5-100 .mu.m. Several methods are known to apply to metallic coating onto a sheet.
For example, one can proceed by laminating a film of aluminum onto the sheet to be coated, but this method is expensive. Alternatively, one can proceed by immersing the sheet in a liquid bath based on aluminum.
When the method by immersion is used, as described in the article in the journal STAHL and EISEN, Vol. 111, No. 12, Dec. 12, 1991, pp. 111-116 (THYSSEN Forschung, Duisburg), notably in FIG. 4 and in the middle of page 112, (incorporated herein by reference) the coating comprises:
an interface or internal layer consisting essentially of one or more alloys based on iron and aluminum, and PA1 an external layer comprising essentially a principal phase based on aluminum, and secondarily, other phases in the form of needles or elongated lamellae dispersed in said principal phase; the article cites the presence of eutectic phases between the solidified aluminum dendrites. PA1 T. Yamada and H. Kawase, presented at the 5.sup.th "IAVD Meeting" in 1989 (IAVD: "International Society for Vehicle Design"). PA1 Y. Hirose and Y. Uchida, in the supplement of the journal "Japan Institute of Metals," No. 3, 1983. PA1 an internal layer 1, applied to the steel 2, consisting essentially of one or more alloys based on iron and aluminum, and silicon, notably a so-called .tau.5 phase and/or a so-called .tau.6 phase. PA1 an external layer 3 consisting essentially of aluminum in the form of large dendrites; these dendrites are often (but not always) saturated with iron and, optionally, silicon in solid solution. PA1 lamellae consisting essentially of silicon, and PA1 needles consisting essentially of an intermetallic phase .tau.6. PA1 an internal layer comprising, consisting essentially of, or consisting of one or more alloys based on iron, aluminum and/or silicon, and PA1 an external layer which comprises, consists essentially of, or consists of a phase based on aluminum and secondarily of other phases in the form of needles or elongated lamellae distributed in said aluminum-based phase, and having a thickness which is larger than or equal to that of said internal layer of alloy, PA1 so that the thickness of the external layer remains larger than or equal to that of said internal layer of alloy, and PA1 so that the projection of the length of all said needles or lamellae in a direction perpendicular to the plane of said external layer is strictly less than the thickness of this layer. PA1 said bath based on aluminum contains at least 6 wt % of silicon, PA1 said bath based on aluminum contains at least 8 wt % of silicon, in which case the proportion of .tau.6 phase in the coating is larger, at the expense of that of .tau.5 phase. PA1 the duration of the thermal treatment, in the phase where said temperature is larger than 570.degree. C., is less than or equal to 15 sec. PA1 an internal layer comprising, consisting essentially of, or consisting of one or more alloys based on iron, aluminum and/or silicon, and PA1 an external layer which comprises, consists essentially of, or consisting of a phase based on aluminum and secondarily of other phases in the form of needles or elongated lamellae distributed in said aluminum-based phase, and having a thickness which is larger than or equal to that of said internal layer of alloy, PA1 the thickness of said internal layer of alloy is less than or equal to 5 .mu.m; this smaller thickness makes it possible to limit the risks of the appearance of cracks, PA1 said coating comprises compounds based on aluminum nitrides intercalated between the steel of said sheet and said internal layer, PA1 the content of free nitrogen of said steel is greater than or equal to 10.sup.-2 wt %. PA1 an internal layer comprising, consisting essentially of, or consisting of one or more alloys based on iron, aluminum and/or silicon, and PA1 an internal layer which comprises, consists essentially of, or consists of a phase based on aluminum and secondarily other phases in the form of needles or elongated lamellae distributed in said aluminum-based phase, and having a thickness which is larger than or equal to that of said internal layer of alloy, PA1 preferably characterized in that, before the shaping step proper of said sheet, said sheet is subjected to a thermal treatment which is adapted so as to increase the temperature of at least the external layer above 570.degree. C. and below 660.degree. C., under conditions, notably of duration, heating rates and cooling rates, which are adapted: PA1 so that the thickness of the external layer remains larger than or equal to that of the internal layer of alloy, and PA1 so that the projection of the length of all said needles or lamellae in a direction which is perpendicular to the plane of said external layer is strictly less than the thickness of this layer.
Since, seen in cross section, lamellae are in the form of needles, it is difficult to distinguish, in practice, needles from lamellae.
The internal layer consisting of an alloy has a fragile behavior, and therefore attempts are generally made to limit its thickness.
To limit the thickness of this layer of alloy, immersion baths are generally used which contain a compound which inhibits alloying between the aluminum and the steel.
Silicon is the most frequent inhibitor of alloying used; to be effective, its concentration by weight must generally be larger than 6% in the immersion bath.
Other known means exist to limit the thickness of this layer of alloy, such as using, before the coating, a slight nitration of the surface to be coated, for example, by conducting recrystallization reheating of the steel to be coated in an atmosphere containing traces of ammonia.
Certain aluminum coated sheets can then be subjected to thermal treatments, either to modify their properties, or even in normal usage (for example: thermal screens); it is also important in this situation not to increase the thickness of the internal layer of alloy appreciably.
To limit this risk of growth of the internal layer of alloy during subsequent thermal treatments, it is known to use types of steel containing sufficient contents of free nitrogen (for example, .gtoreq.10.sup.-2 wt %); these steels can be renitrided steels; in this regard, reference is made to the following articles, all incorporated herein by reference:
As diagrammatically represented in FIG. 1, when the coating is applied to the immersed material, the coating that one obtains is divided into two principal superposed layers:
The internal layer can be subdivided into several sublayers comprising still other phases; at the interface between the internal layer 1 and the steel 2, one can sometimes find a sublayer comprising the following phases: a so-called .eta. phase (Fe.sub.2 Al.sub.5), a so-called .theta. phase (FeAl.sub.3), and one or more phases based on aluminum nitride; the thickness of this sublayer in generally does not exceed 1 .mu.m.
At the level of the external layer 3, when a bath is used which contains silicon, phases are generally observed which are richer in silicon and/or iron that the aluminum dendrites; these phases often present an elongated lamellar or needle-shaped form.
As phases 4 with elongated form, the following were identified, for example:
The external layer can also comprise alloy phases based on aluminum, silicon and iron, notably of eutectic composition with a low melting point.
The phase .tau.5 has a hexagonal structure; it is sometimes called .alpha..sub.H or H; the iron content of this phase is generally 29-36 wt %; the silicon content of this phase is generally 6-12 wt %; the remainder consists essentially of aluminum.
The .tau.6 phase has a monoclinal structure; it is sometimes called .beta. or M; the iron content of this phase is generally 26-29 wt %; the silicon content of this phase is generally 13-16 wt %;
the remainder consists essentially of aluminum.
Table I below recapitulates possible compositions and melting temperatures of the phases present in the coatings which one obtains after immersion in an aluminum coating bath (whose composition and melting temperature are specified in the same Table).
The .tau.6 phase predominates when the bath contains more than 8 wt % silicon; the inclusions of .tau.6 phase present an elongated form, whereas the inclusions of .tau.5 phase generally have a globular shape.
It has been observed that steel sheets coated with an internal layer of alloy based on iron, aluminum and/or silicon and an external layer consisting essentially of aluminum exhibited poor resistance to corrosion after deformation.
Indeed, a deformation, such as a folding, generally causes cracks which open at the surface of the metallic coating; these cracks decrease the corrosion resistance of the steel.
TABLE I Composition of the Phases of the Coating Composition: wt % Al Si Fe Melting temperature Bath &lt;91 &gt;6 3 675.degree. C. (saturation) (T .degree. C. immersed) Eutectic 87 12.2 0.8 =577.degree. C. Al dendrites &gt;98 .ltoreq.1.5 &lt;0.5 .about.660.degree. C. Si lamellae Majority component silicon 1412.degree. C. .tau.6 needles 55 14 31 &gt;577.degree. C. .tau.5 phase 55 to 62 6 to 12 31 to 36 &gt;577.degree. C.